1. Field of the Invention.
This invention relates to motor control circuits, and more particularly, to a proportional motor control circuit which controls the direction of rotation of the motor shaft as a function of changes in a sensed parameter.
2. Description of the Prior Art.
Motor control circuits are employed in analog control systems for controlling a motor which controls system apparatus to maintain a variable system parameter at a predetermined set point value. The motor control circuit responds to an input signal representing deviations of the parameters from the set point to supply drive signals to the motor, which controls apparatus of the system to correct for the error.
By way of example, the motor control circuit may be used in environmental control systems to control the energization of a motor which drives a damper actuator which controls the mixing dampers located in the air ducts supplying warm and cool air to an area or room. The damper actuator is driven by the motor to position the dampers as a function of the temperature in the area as sensed by a temperature sensor. The motor effects repositioning of the dampers to permit warm or cool air to be supplied to the area as needed to compensate for changes in temperature.
In such applications, a reversible motor is generally required to permit opening and closing of the dampers. Thus, the motor control circuit must provide bi-directional drive to the motor to compensate for positive or negative deviations from the set point. Bi-directional drive is generally provided through the use of first and second level detectors which control the motor in response to changes in error signal, indicative of variations in temperature, relative to a reference signal as may be provided by a follow-up potentiometer driven by the motor. One of the level detectors responds to an increase in the error signal relative to the reference indicative of an increase in temperature to enable as associated drive circuit to apply a positive energizing potential to the motor. The other level detector responds to a decrease in the error signal relative to the reference indicative of a decrease in temperature to enable an associated drive circuit to apply a negative energizing potential to the motor. The level detectors operate independently of one another and have respective positive and negative turnon thresholds established by the supply voltage which energizes the control circuit. Thus, proper operation of the circuit is dependent upon balancing of the circuit to provide equivilant positive and negative threshold levels for the level detectors and upon maintaining a constant supply voltage output. For example, assuming the level detectors are initially set to provide symmetrical operation for a given supply voltage, then should the supply voltage decrease for any reason, the sensitivity of one of the level detectors may increase while that of the other level detector may decrease. Such condition is undesirable in environmental control systems wherein the temperature should be maintained at the set point value and, in the event of a change, the temperature should be returned to the set point quickly.
Therefore, it would be desirable to have a motor control circuit for providing bi-directional drive to a motor and which responds quickly to correct for deviations of the system parameter from the set point.